To carry out image forming, a general electrophotographic apparatus such as a digital photocopier first converts an analog image signal, which is supplied from an image input device such as a scanner, into a digital signal, and the digital signal is then subjected to various processings for digital signals: signal conditioning, image area division, color compensation, black generation, variable power zooming process etc. Then the signal is further subjected to filtering and intermediate tone correction, before outputted as an output image signal.
FIG. 10 shows a control block diagram showing an image processing operation in a conventional digital photocopier. The digital photocopier includes an input signal conditioning section 110, an image area division section 120, a color correction/black generation section 130, a variable power zoom section 140, a spatial filter processing section 150, and an intermediate tone correction section 160, a pixel counting section 170, and a total toner consumption calculating section 180. The foregoing various signal processings are carried out by these sections.
With reference to a flow chart in FIG. 11, the following explains an image processing operation in such a digital photocopier.
First of all, an image of a document is scanned by a scanner or the like (step S101) and is supplied to an image processing device. The analog signal is first converted into a multi-valued digital signal. The digital signal is then supplied to an input signal conditioning section 110, and subjected to various processings such as pre-processing for the subsequent image processing, gamma correction for image adjustment, various conversions etc. (step S102)
The image signal is next supplied to the image area division section 120 which classifies the image data by the type of image, and generates an identification signal (area identification signal) which indicates the area type (step S103). Text area and dot picture are typical examples of the image area. The area identification signal is used to enable separate processings for the respective areas in the spatial filter processing section 150 at a later stage, for example, smooth filtering for a dot area, and edge enhancement filtering for a text area. The area identification signal is also used in conversion of gamma characteristic of intermediate tone into another characteristic with more intense density difference, in the intermediate tone correction section 160 at the next stage.
The color correction/black generation section 130 carries out color correction/black generation (step S104) of a signal. The color correction/black generation is required in a apparatus capable of color image forming. With this processing, an RGB image signal transmitted from the image area division section 130 is converted into a CMYK (cyan, magenta, yellow, black) image signal, which is a final state of signal, and now is ready to be outputted.
The CMYK image signal generated in the color correction/black generation section 130 is subjected to variable power zoom process in the variable power zoom section 140 (step S105), and then supplied to the spatial filter processing section 150 where the CMYK signal is subjected to spatial filtering, that is, an appropriate spatial filtering selected from a spatial filter table according to the area identification signal or setting of image mode (step S106). The spatial filter table is a group of tables of coefficients used for spatial filtering, and one of them is arbitrarily selected according to the circumstances.
The intermediate tone correction section 160 corrects an intermediate tone gamma characteristic of the signal so as to correct the output characteristic in the engine section (step S107).
Further, the resulting signal is supplied to the pixel counting section 170 where each CMYK signal is weighted on pixel basis, and the gradation data is added to the counter (step S108). As a result, an output image signal is transmitted to the engine output side of a LSU or LED (step S110). The total toner consumption calculating 180 calculates a toner consumption quantity for each color out of a gross pixel value (gradation value) counted by the pixel counting section 170 (step S109). The toner consumption quantity thus figured out is used for “toner-near-end” detection or accumulation of toner consumption quantity data.
One of the controls carried out by the engine part of the digital photocopier is a process control. Some process conditions in an electrophotography, such as charging potential, exposure level, toner density compensation quantity, development bias, transfer voltage, fixing temperature, fixing pressure, process speed etc., are adjusted so as to avoid degradation of a photoconductor, developer etc. by time. In this way, the toner density, image output etc. become constant throughout the whole life of an apparatus. Such an adjustment is called a process control.
FIG. 12 shows a flow chart schematically showing a toner density control, which is carried out by the engine part of the apparatus as a part of process control. This toner density control is carried out to determine a control value of a toner density sensor in reference to the value of a life counter or an environment sensor (Step S111, Step S112), which control value is used for ON/OFF control as to whether the toner is supplied. More specifically, if the toner density is low (Yes in the step S113), “ON” is selected and the toner supply is carried out (step S114), so that the toner density is kept constant.
FIG. 13 is a flow chart schematically showing an intermediate tone gamma correction by way of toner patch, which finds conditions to determine a control parameter value in the process control. In this intermediate tone gamma correction, a toner patch of an intermediate pattern (tone) having a fixed input value is formed on a photoconductor or on a transfer belt, and a scanning device such as an optical sensor detects an quantity of reflection light from the toner patch.
To be more specific, calibration of optical sensor is carried out in Step S121, and a charging potential, an quantity of light, and a development bias (and transfer voltage, if necessary) in creating a solid image are determined (step S122). In this manner, the density condition of the solid image is adjusted. Then, a toner patch of an intermediate tone having a fixed input value is formed on a photoconductor or on a transfer belt under a density between the density of the solid image and no-image state (step S123). Then the quantity of reflection light from the toner patch is detected by an optical scanner. Next, the output value of the optical sensor is compared with a reference target value in Step S125, so as to find a correction quantity. Then, in Step S126, the existing intermediate gamma correction table is modified according to the correction quantity. In this way the intermediate gamma characteristic is kept constant.
The following more specifically explains the details of calculation of the foregoing toner consumption quantity. Note that, the following processing is performed for each of Cyan, Magenta, Yellow, and Black (for each of the CMYK input signals).
The pixel counting section 170 carries out a pixel counting operation (described later) with respect to a multi-valued image expressed by an input image signal. As shown in FIG. 10, the pixel counting section 170 includes counting means 171, weighting calculation means 172, a weighting coefficient table 173 and accumulating means 174.
The counting means 171 counts the gradation data of a multi-valued image (for example, a multi-gradation image of 16 or 256 gradation levels) for each pixel. More specifically, the counting means 171 counts an input signal value (gradation value, e.g. an input signal value of 0-15 levels (16 gradation levels)) for each of the pixels constituting a multi-valued image.
As the counting means 171 counts the gradation data of each pixel, the weighting calculation means 172 weights the pixel. More specifically, the weighting calculation means 172 first finds a weighting coefficient corresponding to the signal input value of the target pixel from the weighting coefficient table 173, and multiplies the signal input value by the coefficient to figure out a pixel count value. The weighting coefficient table 173 stores plural weighting coefficients for respectively corresponding to plural signal input values. In this manner, with the counting means 171, the weighting calculation means 172 and the weighting coefficient table 173, the pixel counting section 170 calculates a pixel count value for each pixel.
Further, the accumulating means 174 accumulates the all pixel count values which have been separately found. More specifically, after the weighting calculation means 172 figures out the pixel count values by multiplying each signal input value by the corresponding weighting coefficient, the accumulating means 174 accumulates the all pixel count values which correspond to the entire pixels of the all input multi-valued images. Then, based on the gross of the pixel count values found by the pixel count section 170, the total toner consumption quantity calculation means 180 figures out a total toner consumption quantity with respect to the all images having been outputted.
TABLE 1GRADATIONWEIGHTINGVALUECOEFFICIENTAREA 10-40AREA 25-81AREA 3 9-123AREA 413-154
In Table 1, the sixteen signal input values which differ in toner consumption are classified into four areas (areas 1 to 4), which are respectively allotted with predetermined weighting coefficients. In the calculating of the pixel count value, one of the weighting coefficients corresponding to the four areas are allotted to each of the signal input values having the values 1-15, so that the signal input values are weighted. According to table 1, the signal input values of 0-4 gradation levels are weighted by a coefficient of 0, the signal input values of 5-8 gradation levels are weighted by a coefficient of 1, signal input values of 9-12 gradation levels are weighted by a coefficient of 3, and signal input values of 13-15 gradation levels are weighted by a coefficient of 4.
FIG. 14 shows correspondence between the signal input values and the weighting coefficients of 4 areas (four divisional areas) in the weighting coefficient table. As shown in FIG. 14, the gross area of the rectangle parts of each area is substantially equal to the area formed by the curved line which shows a toner consumption characteristic. According to this, the toner consumption quantity may be estimated by the gross of the weighted pixel count values.
There are many conventional techniques for calculating the toner consumption quantity, as disclosed in Japanese Laid-Open Patent Application Tokukai 2004-163553 (published on Jun. 10, 2004), Japanese Laid-Open Patent Application Tokukaihei 10-333419 (published on Dec. 18, 1998), Japanese Laid-Open Patent Application Tokukaihei 10-239979 (published on Sep. 11, 1998), Japanese Laid-Open Patent Application Tokukai 2001-296706 (published on Oct. 26, 2001), and Japanese Laid-Open Patent Application Tokukai 2004-309533 (published on Nov. 4, 2004). Also, the applicants of the present invention previously made an invention relative to the present invention, which is disclosed in Japanese Laid-Open Patent Application Tokukai 2006-023392 (published on Jan. 26, 2006: corresponding to US Patent Application No 2006007509 (A1)).
These conventional electrophotographic apparatuses such as digital photocopier, however, have the following drawback.
As described, when the toner consumption quantity of an output image is estimated through pixel counting, the gradation values have been weighted in accordance with a weighting coefficient table having predetermined fixed weighting coefficients. However, as shown in FIG. 14, weighting with such a weighting coefficient table may raise a significant difference between the value of weighting coefficient allotted to the signal input value from the weighting coefficient table and the value on the curved line which shows a toner consumption quantity characteristic of the signal input value. This typically happens to the weighting coefficients corresponding to the signal input values 4, 5, 8, 9 and 12. This problem decreases accuracy of estimation of toner consumption quantity based on the gross of the pixel count values.
Such a problem may be solved by using a weighting coefficient table having the same number of coefficients as the number of the gradation values of the input signals, that is, the table has the coefficients individually corresponding to the signal input values. The weighting coefficients in this case are shown in FIG. 15. With this table, the actual toner consumption quantity characteristic and the toner consumption quantity estimated based on the pixel counting come closer. In FIG. 15, the weighting coefficients are determined according to the toner consumption quantity characteristic expressed by a curved line D, but the characteristic may form the curved line D or the curved line E, depending on the apparatus type or the lives.
However, the weighting coefficients are each determined as a corresponding value to the signal input value of the pixel concerned, and therefore they are determined with no account of the input signal values of the peripheral pixels. Even with the same quantity of signal input value (gradation data) of the pixel, an electrostatic latent image on a photoconductive drum is developed differently depending on the signal input values of the peripheral pixels. For example, when a pixel is irradiated with a light beam from an exposure device under a certain condition determined for the gradation value, the quality of the electrostatic latent image varies depending on the exposure condition of the peripheral pixels. Further, this variation also causes variation in quantity of toner adhesion to the electrostatic latent image. This indicates the fact that the toner consumption quantity of a pixel is under influence of the signal input values of the peripheral pixels.
This conventional drawback results in inaccuracy of estimation of toner consumption quantity. Therefore there has been some error between the estimation result and the actual toner consumption quantity.